Steel sheet may be processed by cold rolling in a rolling mill to increase its strength. This is done by passing the sheet between a pair of rollers spaced by a distance less than the thickness of the sheet. In practice, owing to bending or expansion of the rollers, the pressure applied to the sheet is not completely uniform in either the length or width direction. This lack of uniformity produces internal compression and tensile stresses in the sheet that vary along its length and/or across its width.
In general, a steel sheet that has a uniform tension distribution will lie flat on a horizontal table if it is unwound and set down with the tension removed. Steel sheet having a non-uniform tension distribution does not generally lie flat, but instead has wavy or buckled portions that correspond to the areas of the sheet rolled with the lowest tension. Thus, the flatness of a sheet may be determined by measuring its tension distribution. Examples of tension measuring devices that make use of this fact can be found, for example, in U.S. Pat. No. 5,537,878 to Sendzimir and U.S. Pat. No. 3,499,306 to Pearson, the disclosures of which are hereby incorporated by reference.
The data produced by such tension sensors may be analyzed by computer to detect tension distributions that are outside normal ranges. However, merely detecting whether tension levels are too high or too low does not give complete information regarding the flatness of the sheet. Prior art devices may allow a sheet to be generally characterized as either acceptable or unacceptable, but provide little useful information as to how the tension varies across the length and width of the sheet. If more details concerning the tension distribution were available, decisions could be made concerning which portions of the sheet might be acceptable for some uses. Moreover, if information concerning the flatness of the sheet were available in real time, an operator might quickly determine what was causing an out-of-flatness condition and take the appropriate steps to correct the problem.
It is therefore desirable to provide a method and apparatus for receiving tension signals from a plurality of sensors measuring tension in a rolled sheet, and providing an output that produces, on a display, in real time, a representation of the flatness of the sheet.